Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data that are readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. As should be understood, to maximize the amount of data that can be stored on a disk surface, the inner and outer diameters of the data track band should be as close as possible to the inner and outer diameters of the disk itself.
Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to generate an electric current that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap that contains the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disk.
As known in the art, the magnetic transducer is mounted by a head structure to a rotary actuator and is selectively positioned by the actuator over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by rotation of the disk. The air bearing surface of the slider has a leading edge and a trailing edge. Typically, in currently used heads, such as, e.g., Transverse Pressure Contour (TPC) heads, two spaced rails are arranged to extend longitudinally along the lateral sides of the air bearing surface, one adjacent each lateral side, from the leading edge to the trailing edge of the surface. The rails provide various pressure effects to cause head flying operation.
Thus, the transducer does not physically contact the disk surface during normal operation of the disk drive. The amount of distance that the transducer flies above the disk surface is referred to as the "fly height". It is a design goal to maintain the fly height of the head at an even level regardless of the radial position of the head.
In modern disk drives, a relatively rigid or hard disk is used as the magnetic medium. The disk comprises a hard substrate such as aluminum. Layers of various materials are applied to the surface of the aluminum substrate by, e.g., a sputtering process to provide layers that are substantially smooth and flat. The surfaces obtained from the sputtering process are designed to facilitate an even fly height for the head. The layered materials include a layer of magnetic material to provide the recording medium for the magnetic transitions representing data.
Typically, the outer diameter of the substrate is slopped at the radial outer end of the disk shape. This is referred to as the rolloff of the disk. Thus, at the outer diameter of the disk, the disk surface is no longer flat and usable to sustain a stable fly height of the air bearing surface of the head. Indeed, the flying behavior of the air bearing surface can become unstable if the head moves too far into the rolloff region of the disk, which can result in contact between the head and the disk surface. Any contact between the head and the disk surface may result in damage to the disk or head leading to early disk drive mechanical failure.
Accordingly, it is important to design the disk drive such that the outer diameter of the data track band is spaced suitably inward from any portion of the disk rolloff region where fly height degradation can occur when reading data from or writing data to. data tracks arranged at the outer diameter of the data track band. However, it is desirable that each disk used in a disk drive have a maximum radius relevant to the rolloff region that is equal to or greater than a preselected threshold radius so as to not impact the radial extent of the data track band beyond an acceptable amount.
To that end, during the manufacture of magnetic disks that are to be used in a disk drive, a check should be made of the rolloff radius of each disk as it moves through the manufacturing process, so as to reject any disk having a rolloff radius less than the preselected threshold value. In this manner, each disk made available for assembly into a disk drive will be able to accommodate a maximum data track band width for a maximized data capacity for the drive, without undesirable fly height instability at the data tracks near the outer diameter of the data track band.
At present, the rolloff radius for a disk used in a disk drive is determined by reference to a "dub-off" value. The dub-off value is defined as the maximum height undulation between two radii of the disk at the outer diameter. However, it has been determined that the dub-off value does not provide adequate information regarding fly height stability for a head positioned at a data track near or at the outer diameter of the data track band. In fact, there is a poor correlation between the dub-off value and fly height performance. Accordingly, the presently known disk measurement procedures do not provide an adequate system or process for achieving a reliable quality control for disks relevant to maximizing data capacity by assuring compliance by each disk with a maximum data band width having fly height stability at the outer diameter of the band.